Obtaining developed images by reacting an electron donating colorless dye (hereinafter referred to as "color former") with an electron accepting compound (hereinafter referred to as "color developer") with heat is well known as described in Japanese Patent Publications Nos. 14039/70 and 4160/68 (corresponding to U.S. Pat. Nos. 2,663,654 and 2,967,785, respectively). Heat-sensitive recording sheets have recently been studied with respect to their use in facsimiles.
Recently, the speed of facsimiles have been increased by obtaining faster printing-out using a heat-sensitive element. Accordingly, the heat-sensitive recording materials are required to respond in a shorter time. Recording sensitivity is increased in general in order to increase this response speed.
The recording sensitivity shows the relationship between heat energy applied to the heat-sensitive recording layer and the image density. When a heat-sensitive system forms a thick color by applying a small amount of energy, the sensitivity is said to be high. When a heat-sensitive system requires a large amount of energy to obtain a sufficient density, the sensitivity is said to be low.
Hitherto, in order to increase the recording sensitivity, efforts have been made to (1) reduce the development temperature by addition of a low melting point heat-fusible substance or by adoption of a low melting point color developer, (2) increase the efficiency of thermal conduction by improving the smoothness of the recording face, (3) increase color density by increasing the amount of the color forming components used and (4) increase development efficiency by granulating the color former or the color developer so as to have a small particle size of several .mu.m. For example, Japanese Patent Application (OPI) 47693/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") has disclosed that the recording sensitivity is increased by making the color former so as to have a volume average particle size of 2 .mu.m or less. However, though the recording sensitivity is improved by finely granulating at least one color former and the color developer, fogging on the background increases. The increase of fog on the background causes the commercial value as the recording sheets to be damaged.